Control systems for power supply circuits are known, for instance for providing a short-circuit protection with the help of external protective devices. Such devices detect an abruptly increasing short-circuit current and the power supply circuit is interrupted in response to such detection. Conventionally, a cyclical control is used especially in all digitally controlled systems. Due to such cyclical control the load current circuit, also referred to as power supply circuit, can be switched off only at the next clock cycle, whereby an extreme load could prevail for time durations that are too long. Such loads can occur, for example due to a short for a duration that is too long to prevent burn-out or accelerated aging of all circuit elements but in particular the power supply switch, the load element, and the supply voltage source.
Furthermore, systems are known which use a signal that can assume the states or levels "high", "low", and "high impedance", e.g. with a tristate transistor as described in Tietze/Schenk "Halbleiter-Schaltungstechnik" (Semiconductor Circuit Technology), 10th Edition, Springer Verlag, 1993, p. 209. There a microprocessor is used to provide these signal levels to determine the logic state of a signal conductor.
U.S. Pat. No. 5,391,932 (Small et al.), issued on Feb. 21, 1995, discloses a transistorized coupling circuit for coupling a d.c. power source to a data transmission line. A circuit arrangement is provided for recognizing an excess current situation. A control signal is generated in response to the excess current situation to switch off the power output stage for a defined length of time and thereafter to switch the power supply on again. The operability of the power supply output stage is not checked, whereby reactivation can be attempted even if the stage is not operable, for example when the short is still present in the circuit. The nature of the cause that interrupted the power supply circuit is not recognized.
U.S. Pat. No. 5,383,086 (Wietelmann et al.), issued on Jan. 17, 1995, describes a system and method for triggering an inductive load in the form of an inductive actuator. The current flowing through the actuator is sensed and a respective voltage is fed back to the control circuit. The control of the power supply stage can also be influenced with regard to its voltage supply and with regard to the operating temperatures to influence the scanning rate of the power output stage. Such a circuit is subject to the same short-comings as mentioned above in that the operability of the power output stage is not checked and the OFF-state is not maintained nor can it be checked what caused the OFF-state.
U.S. Pat. No. 5,352,868 (Denen et al.), issued on Oct. 4, 1994, discloses a resistance feedback controlled power supply circuit, wherein the power supply to a load with an ohmic power consumption is subject to a temperature control. The load is scanned with a high frequency alternating voltage while the temperature of the load is measured with the aid of a direct current voltage passing through a resistor.
U.S. Pat. No. 5,262,680 (Hu), issued on Nov. 16, 1993 discloses a switch control circuit for eliminating intermittent ON/OFF conditions on a power supply switch. Three comparators are used in the circuit for controlling a solenoid power supply switch which connects the power supply only to the load if certain power supply voltage levels are met.